Rapid and extensive enlargement of the anterior latissimus dorsi muscle of the Japanese quail can be induced through stretch by the addition of a load corresponding to 10% of the bird's body mass to one wing. We have demonstrated a mean muscle mass increase of 181%, a 51% increase in total fiber number and a significant increase in small slow-beta fibers that were < 500 mu m2, relative to the intra-animal control after 30 d of stretch. Our pilot data indicate that new muscle fibers begin to appear within the first week of stretch. Two possible mechanisms which might contribute to increases in fiber number in adult muscle are; 1) activation of muscle precursor cells and formation of nascent fibers and 2) fiber branching where two or more daughter fibers are formed from a single parent fiber. The occurrence and relative importance of these mechanisms to fiber proliferation has not been previously examined in adult muscle. This study will focus upon the mechanisms of new fiber formation contributing to increases in muscle mass during the initial 7 days of stretch. The rate and frequency of fiber proliferation will be determined by direct counting of fibers after nitric acid digestion of connective tissue. The period of peak DNA synthesis of muscle precursor cells and its relationship to the period of peak fiber proliferation will be determined by autoradiographic studies. Evidence for possible migration of satellite cells to the interstitium will be obtained by ultrastructural examination of labelled cells. Immunocytochemical identification of myosin isoforms by monoclonal antibodies will be used to determine if new small fibers express a non-adult myosin isoform (i.e. embryonic or neonatal). The importance of branched fibers to new fiber formation will be determined by immunocytochemical and ultrastructural experiments using serial reconstructions of muscle cross-sections. Future studies will examine the contributions of these mechanisms to muscle hypertrophy which exceed 7 days, determine if the new fibers are functionally innervated, and to elucidate whether the new fiber population remains stable after the hypertrophic stimulus has been removed, or if fiber number returns to its pre-stimulated level. Long- range objectives are to understand the control mechanisms contributing to new fiver formation. Examining the cellular mechanisms involved in fiber proliferation of adult muscle is essential for understanding processes involved in muscle mass increases for rehabilitation of disuse and muscle wasting diseases, improvements of age-induced muscle atrophy and muscle mass increases for sports related events.